The present invention relates to a device used for the photorefractive keratectomy of the eye, comprising an UV laser beam (UV) for ablating parts of the cornea, and a fixation light beam which has a wavelength in the visible region and which is directed onto the eye.
In photorefractive keratectomy (PRK) a defective vision of the human eye is corrected in that part of the cornea is re-shaped. A special PRK method, which is substantially gaining significance at present, is LASIK. According to the LASIK method, a flap is cut into the cornea and opened up. Subsequently, an UV laser beam (normally an Excimer laser beam having a wavelength of 193 nm) is directed onto the exposed parts of the cornea (parts of the cornea uncovered by the flap) so as to ablate material there. After the desired ablation, the flap is clapped down and heals with the cornea.
The present invention relates to PRK in general and to the LASIK method in particular.
In the case of PRK and LASIK, respectively, the ablation profile, i.e. the local distribution of the thickness of the corneal regions to be ablated, depends on the nature of the visual defect to be corrected. When the visual defect to be corrected is myopia or hyperopia, the ablation profile is rotationally symmetric; when astigmatism is to be corrected, the ablation profile is asymmetric. The latter also applies to the correction of higher-order visual defects (wavefront correction, aberration correction).
For all the above-mentioned corrections it is, however, normally necessary to define an axis with regard to which the ablation is centered. The ablation according to the ablation profile is carried out according to algorithms known to those skilled in the art, i.e. the UV radiation is directed onto the cornea to be modified, in a temporal and local distribution corresponding to the algorithm. The algorithm needs an axis which defines the centre of the ablation profile, i.e. the axis extends through the cornea surface to be treated and the ablation profile is represented with regard to the point of intersection between this axis and the cornea surface. This permits centering of the PRK and LASIK methods, and this is what matters in the present invention.
The centering of corneal surgical procedures is made more difficult by the circumstance that the human eye is not a centered optical system, i.e. a common optical axis of the cornea, anterior chamber, lens, vitreous body chamber and fovea of the eye does not exist. H. UOZATO and G. L. GUYTON describe quite generally the problem of centering in the case of corneal surgical procedures, cf. the American Journal of Ophthalmology 103:261-275, March 1987. This prior art is assumed to be known in the following. In this prior art it is suggested that the centre of the entrance pupil of the eye, but not the so-called xe2x80x9cvisual axisxe2x80x9d, should be used for centering corneal surgical procedures and it is explained how this can be done. A so-called optical xe2x80x9cline of sightxe2x80x9d is defined, which connects the fixation point to the centre of the entrance pupil. As has already been mentioned, the above-defined line of sight is to be understood optically but not geometrically. The fixation point corresponds to the above-mentioned fixation light beam, i.e. it is the point fixated by the patient. H. UOZATO et al. (see above) suggest that the point of intersection between said line of sight and the front face of the cornea should be used as a centre for the corneal surgical procedure. This prior art is taken as a basis in the present invention, i.e. the point where the line of sight passes through the front face of the cornea is chosen as the centre of refractive correction. In the LASIK method the term xe2x80x9cfront face of the corneaxe2x80x9d is to be interpreted such that the exposed surface of the cornea (which can be located e.g. in the stroma) defines the above-mentioned piercing point (point of passage).
Theoretically, the following course of action could be taken for determining said piercing point: the line of sight connects the fixation point to the fovea. The fixation light forms a reflex (i.e. it is reflected) on the front face of the cornea, the so-called scattered light/Fresnel reflex. Hence, this scattered light/Fresnel reflex starts from the piercing point and permits therefore a determination of the position of said piercing point. The scattered light/Fresnel reflex should not be confused with the Purkinje-Sanson image (cf. the prior art cited at the beginning). It follows that, making use of this method, the scattered light/Fresnel reflex of the fixation light beam could be chosen as a means of marking the ablation centre. The image of the scattered light/Fresnel reflex has, however, an extremely weak luminous intensity and is, in addition, swamped out by the Purkinje-Sanson image so that measurements according to this method would be extremely difficult.
WO 95/27453 describes the use of an infrared light beam which is directed onto the cornea coaxially with the actual ablation beam (Excimer laser beam). Prior to each ablation pulse, the position of the corneal front face reflex of this IR beam is determined by means of a so-called eye tracking camera. Through image evaluation, a comparison is carried out between a desired position and an actual position of this reflex relative to a reference point of the eye tracking. If the actual position determined deviates from the desired position, an attempt will be made to equalize this difference by means of a control element (the scanner). If this cannot be done, the ablation pulse will not be released. In this prior art, a localization of the point at which the fixation light beam passes through the front face of the cornea does not take place. It follows that, in the case of this prior art, it is also impossible to choose this point as a centre for the subsequent refractive correction of the cornea.
WO 95/28879 and WO 95/28989 describe methods for determining the eye position by means of four IR diodes which are reflected with different intensities on an optical interface on or in the eye. The position of the eye is calculated on the basis of the measured reflected intensities of the radiation.
It is the object of the present invention to provide a device and a method by means of which centering for photorefractive keratectomy, especially LASIK, can be carried out in a reliable manner.
According to the present invention, this is achieved by means of a device of the type specified at the beginning, which comprises, in addition to the UV laser beam used for ablating the cornea and the fixation light beam which must be in the region visible to the patient, a centering light beam extending coaxially with the fixation light beam and having a wavelength different from that of the fixation light beam, means being provided for measuring the scattered light/Fresnel reflex of said centering light beam on the front face of the cornea.
In the LASIK method, the xe2x80x9cfront face of the corneaxe2x80x9d is the cornea surface that is exposed when the flap has been opened up.
Due to the fact that the wavelength chosen for the centering light beam is different from that of the fixation light beam, it is possible to separate the two beams from one another when the reflex is being measured and to discriminate them so that the fixation light will no longer disturb the measurement.
The centering light beam has preferably a wavelength in the infrared region, especially in the range from 800 to 1,100 nm.
A specially preferred embodiment of the invention is so conceived that a camera is used for measuring the position of the scattered light/Fresnel reflex of the centering light. In devices used for the photorefractive keratectomy of the eye, such a camera is often already provided for other reasons, in particular for so-called eye tracking. The eye tracking technique is described e.g. in DE 197 02 335 C1; also this prior art is assumed to be known in the following. According to this prior art xe2x80x9ceye trackingxe2x80x9d is carried out as follows: when performing photorefractive keratectomy, and in particular LASIK, it is important that the relative position of the ablating laser beam and the eye is precisely known. In the case of optical fixation, which is preferably used also in the present invention, the patient is requested to look precisely at the point defined by the fixation light beam so that the eye will maintain the same position during the whole surgical operation. However, the patient does not normally succeed in doing so, at least not with sufficient reliability so that movements of the eye occur which may seriously impair the whole ablation process. xe2x80x9cEye trackingxe2x80x9d means that the movements of the eye are determined so that the laser beam used for the ablation can then be controlled (caused to follow) in accordance with the eye movements. The above-cited prior art describes how during xe2x80x9ceye trackingxe2x80x9d images of the eye are recorded by means of a camera (solid state camera, CCD) and processed in rapid succession. A change in the position of the eye can be determined from successive images, and the ablation laser beam is then caused to follow in accordance with the eye movement with the aid of suitable beam control means (e.g. a galvanometric scanner). The cited prior art also teaches that, for determining a movement of the cornea, the centre of the pupil should be determined and the movement thereof should be ascertained. In the case of this photographic evaluation, the eye is normally illuminated with IR radiation and the image of the eye, in particular of the iris with the pupil, is recorded with the camera for the purpose of evaluation. This IR radiation, by means of which the eye is illuminated for the above-mentioned purpose, can be used together with the present invention and will neither be specially mentioned nor described in the following. Even though the centering light beam has wavelengths in the IR region, the IR rays reflected by the eye can be deflected by means of suitable partly reflecting mirrors and separated from the reflections of other wavelengths in such a way that the position of the pupil, especially the geometrical centre thereof, as well as the position of the scattered light/Fresnel reflex can be determined by means of the camera and a computer connected thereto. Hence, it is possible to establish a correlation between the ablation centre to be located and the geometrical centre of the pupil, i.e. to form e.g. the vector from the geometrical centre of the pupil to said reflex and to define subsequently, on the basis of this vector, the ablation centre for the execution of the ablation algorithm during the ablation. The measuring accuracy with regard to the vector can then be improved e.g. by forming an average value over a plurality of individual measurements. It is also possible to determine the vector for different pupil diameters and to carry out averaging over these subsequently.